The present invention relates to a gate turn-off thyristor having emitter, control base, main base and counter-emitter zones.
A thyristor is a semiconductor switch which permits the switching on of a load current by means of a relatively low control current. In normal thyristors switching off is effected in that the load current is lowered to below a minimum value, the so-called holding current. However, so-called gate turn-off thyristors are known in which switching off as well as switching on are effected via the control current, the control current being directed in the opposite direction for switching off than for switching on of the thyristor. The switch-off behavior is characterized by the turn-off gain which is defined as the ratio of the anode current to the required negative control current. In the presently known embodiments of gate turn-off thyristors, however, a relatively large control current, which typically is 10 to 30% of the load current carried by the thyristor in the switched on state is required for switching off the thyristor. In particular, a considerable forward voltage drop must be tolerated which amounts to several volts for a current load of several 10 A/cm.sup.2. Furthermore, the holding current also becomes very high in such thyristors. Such devices are described, for example, in the U.S. Pat. No. 3,210,563 issued Oct. 5th, 1965 to T. C. T. New and in the Canadian Pat. No. 774,273 issued Dec. 1967 to Moyson et al.
A thyristor usually has four zones of alternatingly opposite conductivity type. The following terminology will be employed hereinafter for the individual zones of the thyristor: the conductive zone provided with the control terminal or terminals will be called the control base zone, the adjacent inner zone will be called the main base zone, the outer zone adjacent the control base zone will be called the emitter zone, and the outer zone adjacent the main base zone will be called the counter-emitter zone.